Electroless copper plating baths are in widespread use in industry for depositing copper on non-conductive plastic substrates. In the manufacture of printed circuit boards, for example, electroless copper baths are used to deposit copper into the holes and/or circuit paths as a base for subsequent electrolytic copper plating. Electroless copper plating is also used in the decorative plastics industry for depositing copper onto the plastic surface as a base for further plating of copper, nickel, gold, silver, or other metals as required. The baths that are predominantly in use today usually contain a soluble divalent copper compound, a chelating or complexing agent for the divalent copper ions, a formaldehyde reducing agent, and various addition agents to make the bath more stable, plate at higher speed, or brighten the copper deposit. In spite of the fact that these baths are highly successful and widely used, the industry has been searching for alternative electroless copper plating baths that do not contain formaldehyde due to its toxic nature.
Hypophosphites have been suggested as a replacement for formaldehyde; however, plating rates of baths containing this compound are generally too slow. Dimethylamine borane alone has also been tried with varying degrees of success; however, there is no commercially successful plating bath to date that uses this reducing agent. The following patents are representative examples of the current state of the art in this area.
U.S. Pat. No. 3,431,120 proposes to use an electroless copper plating bath containing dimethylamine borane reducing agent with glucoheptanoic acid as a complexing agent. The pH range of these baths ranges from 3.5 to 7.
U.S. Pat. No. 3,870,526 discloses an electroless copper plating bath containing a dimethylamine borane reducing agent and ethylene diamine tetra-acetic acid ("EDTA") as the chelating agent, plus ammonium hydroxide to adjust the pH to the range of 8 to 11 (normally about 10.7).
U.S. Pat. No. 4,138,267 discloses an electroless copper bath having a pH between 12 and 14, sodium borohydride as the reducing agent, and various hydroxy substituted ethylene diamines as a chelating agent.
U.S. Pat. No. 4,143,186 proposes to use an electroless copper plating bath using dimethylamine borane reducing agents and various complexing agents, such as tartrates, acetates, glycolic acid, pyrophosphates, phosphates, EDTA, nitrilo-triacetic acid ("NTA"), ethylene diamine, triethylene tetra-amine, gluconic acid or gluconates at a pH range of 4 to 7.5. Triethanolamines are also mentioned as possible complexing agents.
U.S. Pat. No. 4,684,550 discloses a formaldehyde-free electroless copper bath using a dimethylamine borane reducing agent, an EDTA complexing agent, thio diglycolic acid as a stabilizer; an adduct of ethylene oxide and an acetylenic glycol as a surfactant and ammonium hydroxide to adjust the pH to the range of between 8 and 11.5.
In all the above electroless copper plating baths, addition agents are used to achieve stability of the plating bath and a bright colored copper deposit. These addition agents are the same as those commonly used in formaldehyde containing electroless copper baths: cyanides, ferrocyanides, various sulfur-containing additives, dipyridyl compounds and certain wetting agents. None of the above prior art electroless copper baths achieved commercial success since they were either too unstable, too slow, emitted too high a degree of ammonium hydroxide fumes, or gave poor coverage, and none of them discloses or teaches to utilize a pH range of between 7.5 and 8. There still exists a wide spread industry need, even to this date, for a formaldehyde-free electroless copper plating bath that does not compromise any of the good characteristics commonly achieved with the commercially used formaldehyde-containing electroless copper baths. These characteristics include a good plating rate, good copper colored deposits, good and complete coverage of the substrate by the deposit, good bath stability, and ease of control.